Water Water Water! Dehydration causes it. But timing is key. I drink plenty of water a couple of hours before sessions, then of course keep lots on hand during. Also watch for saliva strings (Yechhh!!) Sounds gross, but if you open your mouth wide and close it several times in front of the mirror, you may or may not see saliva strings that link the top and bottom of your mouth. Seeing them is an indication that you're dehydrated. Drinking more water will clear this up. (My sister is a Dental person and told me that little nugget!)
Here's something that works for me, though it might not be for everyone.... Eat orange peels! I'm fortunate in that I have two mature, fruit laden orange trees in my back yard here in southern CA, but.... They're bitter as all get out, but also very high in pectin. One other side benefit (how do I put this delicately?): they're a great source of roughage--you'll never need a wheatgrass shot again! If you're using store-bought oranges make sure you wash the peel thoroughly (you might even want to try a scrubbing them with hydrogen peroxide to remove traces of pesticides). They're definitely an acquired taste, but I've really come to, um, enjoy them!